1. Field of the Invention
The invention relates to a process for the metered addition of a fluid medium into a material suspension stream of a headbox in which the material suspension stream in the headbox is distributed across the machine width, fed into a plurality of turbulence-producing conduits, and supplied to the headbox nozzle. Furthermore, a total pressure loss ΔPtotal is caused in the region of the turbulence-producing conduits, including at least one intake pressure loss ΔPIntake created at the entrance to the conduits followed by a graduated pressure loss ΔPgrad created at a graduated pressure loss area defined by at least one change in cross-section inside the conduits. Furthermore, the metered addition of a fluid medium occurs in the region of the conduits, between the conduit entrance and the conduit exit.
The invention also relates to a headbox for a paper or cardboard machine having at least one material suspension supply running the width of the machine, at least one turbulence producer having a plurality of conduits through which the material suspension flows, a plurality of metered addition points for a fluid medium in the region of at least one of the conduits and an attached headbox nozzle.
2. Discussion of Background Information
DE 35 14 554 A1, describes a conventional headbox design, wherein FIG. 6 shows a headbox having a suspension supply across the width of the machine and a turbulence producer having several conduits through which a material suspension flows. The conduits have a plurality of metered addition points for introducing a fluid medium into one part of the conduits. Moreover, a headbox nozzle is attached to the turbulence producer.
The German patent also discloses a corresponding process for the metered addition of a fluid medium into a material suspension stream of a headbox in which the material suspension stream distributed across the machine width utilizes a plurality of turbulence-producing conduits which are fed by these to the headbox nozzle. In the region of the turbulence-producing conduits, a total pressure loss is produced that arises from at least the intake pressure loss in the conduits and the subsequent graduated pressure losses from cross-sectional change inside the conduits. The metered addition of the fluid medium occurs in a part of the conduits between the conduit entrance and the conduit exit.
In this conventional design, the main pressure loss of the turbulence-producing conduits occurs in the entrance region of the turbulence inserts, i.e., before the metered addition points of the fluid medium. As a result of this arrangement, changes in the flow rate through the conduits during the metered addition affect the basis weight cross profile and fiber orientation cross profile in a negative manner. One attempt to solve this problem is shown in FIG. 6 of the application cited above, in which a fluid is sprayed into the lateral distributor of the headbox near the entrance opening of a turbulence tube, i.e., before the location of the main pressure loss. This design provides for a very small influence of the metered addition on the total flow rate of the turbulence conduits, but it also has the disadvantage that no clear assignment of the added fluid to a certain turbulence conduit is given, with the working width of the metered addition also depending on the amount of the fluid added.